Magnesium-containing alloys are finding an increasing number of applications, especially where lightweight structural members are required. The need for lightweight structural materials is particularly acute in the automotive industry where fuel savings, reduced exhaust emissions and other advantages are obtained from reduced vehicle mass. While the need for the use of lighter materials of construction in the automotive industry has long been apparent, the relatively high cost of magnesium and its alloys has been a deterrent to their usage.
Most of the world's supply of magnesium metal is obtained by the electrolytic dissociation of magnesium chloride to magnesium and chlorine. The magnesium chloride is a component of a homogeneous molten electrolyte that also comprises one or more alkali metal chloride salts and/or alkaline earth metal chloride salts. Examples of such additional salt ingredients of the magnesium chloride electrolyte are sodium chloride, potassium chloride, lithium chloride, calcium chloride and barium chloride. Sometimes a small amount of a fluoride salt such as calcium fluoride or magnesium fluoride is also included in the electrolyte composition. A magnesium-producing cell also includes one or more anodes which are typically made of a suitable form of carbon such as graphite. The cathode may be made of steel or may consist of a molten metal layer in contact with the electrolyte. When a direct electrical current is passed through the magnesium chloride-containing electrolyte in such a cell, chlorine is evolved at the anode and magnesium metal collected at the cathode. The magnesium chloride thus consumed is replaced by a suitable feedstock of magnesium chloride, and the cell is operated on a continuous basis until the accumulation of impurities or other event requires that it be shut down.
A significant element of the cost of the electrolytic production of magnesium is the preparation of the magnesium chloride for addition to the electrolyte bath as the original magnesium chloride content is decomposed into magnesium and chlorine. The magnesium chloride must be provided in a form suitable for use in the above-described electrolytic bath. However, pure, dry magnesium chloride is not easy to obtain.
Magnesium is the world's most readily available metal. The ocean is an enormous reservoir of magnesium, and there are abundant salt deposits and other mineral deposits of magnesium. Sea water, salt lake water and brine waters contain magnesium chloride (MgCl.sub.2). Mineral deposits include magnesite-magnesium carbonate (MgCO.sub.3), dolomite, a mixture of magnesium carbonate and calcium carbonate (CaCO.sub.3.MgCO.sub.3), carnallite, a combination of magnesium chloride and potassium chloride (KCI.MgCl.sub.2 .multidot.6H.sub.2 O) and brucite, a magnesium hydroxide (Mg(OH).sub.2).
The design or specification of a process for the production of magnesium metal is affected in large measure by the selection of a feedstock material and its adaptation to a form of magnesium chloride for addition to the electrolyte bath. While magnesium oxide may be cheaper to obtain from mineral deposits than magnesium chloride from seawater or brine, magnesium oxide has been difficult to use because of the absence of a suitable electrolyte solvent for the homogeneous electrolytic decomposition of the magnesium oxide. At the present time, most commercial processes except for the Dow process use anhydrous magnesium chloride. Textbooks describe the Dow process as using a partially dehydrated magnesium chloride that is nominally about 73 percent by weight magnesium chloride. The various separation and purification processes in use for obtaining MgCl.sub.2 including the Dow process are described, for example, in the Encyclopedia of Chemical Technology, Kirk-Othmer, 3rd Edition, Volume 14, at pages 576-583.
In each of the feedstocks, it is difficult to prevent the buildup of magnesium oxide in the electrolytic magnesium production cell. Magnesium oxide is either added as an impurity with the magnesium chloride, it is formed when the partially dehydrated magnesium chloride is melted and added to the bath, or it is generated by oxidation of MgCl.sub.2 in the bath itself. In any case, the efficiency or duration of the operation of a magnesium cell is reduced by the presence or formation of magnesium oxide as a separate, sludge-forming contamination in the magnesium chloride-containing electrolyte.
The efficiency of all current magnesium production processes could be improved by the utilization of a practice that converts magnesium oxide to magnesium chloride in the magnesium production cell or for use in the cell.